LeetCode-Convert BST to Greater Tree

Description：

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /   \
           2     13

Output: The root of a Greater Tree like this:
             18
            /   \
          20     13
           

題意：對于一顆二叉搜尋樹，對于每個結點累加上這顆樹中比目前結點大的值；

解法：我們知道一顆二叉搜尋樹中，root.left.val < root.val < root.right.val；是以，問題就轉化為了如何周遊一顆二叉樹，周遊次序為root.right -> root -> root.left；同時，我們用一個變量記錄周遊過程中的所有值得和，這樣在傳回計算上一個值時進行累加得值就是這個統計值；

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int sum = 0;
    
    public TreeNode convertBST(TreeNode root) {
        if (root == null) return root;
        convertBST(root.right);
        sum += root.val;
        root.val = sum;
        convertBST(root.left);
        return root;
    }
}